Prism-Coupled Light Emission from Tunnel Junctions

We have observed completely $p$-polarized light emission from smooth $\mathrm{A}\mathrm{l}-\mathrm{A}\mathrm{l}{\mathrm{O}}_x-\mathrm{A}\mathrm{u}$ tunnel junctions placed on a prism coupler. The angle and polarization dependence demonstrate unambiguously that the emitted light is radiated by the fast-mode surface plasmon polariton. The emission spectra suggest that the dominant process for the excitation of the fast mode is through conversion of the slow mode to the fast mode mediated by residual roughness on the junction surface.